1. Field of the Invention
The invention relates generally to the field of marine seismic surveying. More specifically, the invention relates to structures for mounting particle motion sensors in a seismic streamer to reduce noise coupled to the particle motion sensor.
2. Background Art
In seismic exploration, geophysical data are obtained by applying acoustic energy to the earth at or near the surface and detecting seismic energy reflected from interfaces between different layers in subsurface formations. The acoustic energy is reflected when there is a difference in impedance between the layer above the interface and the layer below the interface. In marine seismic exploration, a seismic energy source, such as an air gun, for example, is typically used to generate acoustic energy. The acoustic energy is reflected back from subsurface interfaces below the water bottom and is detected by sensors deployed in the water or on the water bottom.
In a typical marine seismic operation, one or more streamers are towed behind an exploration vessel at a water depth between about six to about nine meters. The streamers are essentially long cables having seismic sensors disposed at spaced apart positions along the cable. Hydrophones are typically used as seismic sensors in marine streamers. A hydrophone is a pressure gradient sensor that converts pressure waves into electrical signals that are typically recorded for signal processing and evaluated to estimate characteristics of the earth's subsurface.
U.S. Pat. No. 7,239,577 issued to Tenghamn et al. and assigned to the assignee of the present invention describes the use in seismic streamers of particle motion sensor assemblies that have particle motion sensors such as geophones. Mounting of particle motion sensor assemblies in a seismic streamer according to methods known in the art includes lacing the sensor assemblies to a wire loom in selected locations along the length of the wire loom in a streamer section (a section being a discrete, separable segment of the streamer cable typically about 75 to 100 meters long). The wire loom, for constructional and operational reasons, is generally placed along a streamer section with a certain amount of slack, and is secured firmly only at the longitudinal position of certain types of spacers at selected positions along the length of the streamer section. In between the spacers the wire loom forms a “snake-like” shape. Therefore, the orientation and specific location of the particle motion sensor assemblies can be affected by the local orientation and shape of the wire loom at each sensor assembly. Although during initial construction of a streamer segment the sensor assemblies and the wire loom are placed in relatively precise locations, due to handling and movement of the segment in subsequent assembly steps, the location sensor assemblies can change. Also, the desired axial orientation of the particle motion sensor assemblies relative to the longitudinal axis of the streamer section can be disturbed. It has been observed that after a streamer section is completed, some sensor assemblies can be pressed against the streamer's outer jacket, or can be in contact with stress members that extend along the length of the streamer section and provide axial strength to the section. The foregoing may result in some types of vibration noise traveling along and across a streamer being transferred to the particle motion sensor assemblies and to a decrease of the sensor assemblies' sensitivity to seismic signals of interest due to improper orientation within the streamer.
There continues to be a need for improved structures for marine seismic streamers using particle motion sensors.